Hypothyroidism can cause tiredness, depression, feeling cold, and weight gain.
The study, published in the Journal of the Endocrine Society, found a higher risk of hypothyroidism with long working hours regardless of the workers' socioeconomic status or sex, even though this common thyroid disorder affects women more than men.
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"Overwork is a prevalent problem threatening the health and safety of workers worldwide. To our knowledge, this study is the first to show that long working hours are associated with hypothyroidism," said lead researcher Young Ki Lee from National Cancer Centre in South Korea.
For the findings, the research team conducted the study using data from 2,160 adult full-time workers who participated in the Korea National Health and Nutrition Examination Survey from 2013 to 2015.
The investigators identified hypothyroidism from records of the participants' thyroid bloodwork.
Hypothyroidism occurred at more than twice the rate in participants who worked 53 to 83 hours weekly versus those who worked 36 to 42 hours each week (3.5 per cent versus 1.4 per cent), the researchers reported.
For each 10-hour increase in the workweek, individuals who worked longer hours had an increased odd for hypothyroidism compared with those who worked 10 hours less (odds ratio 1.46).
The researchers stressed the need for further studies to determine whether long working hours cause hypothyroidism, which is a known risk factor for heart disease and diabetes.
"If a causal relationship is established, it can be the basis for recommending a reduction in working hours to improve thyroid function among overworked individuals with hypothyroidism," Lee said
"Additionally, screening for hypothyroidism could be easily integrated into workers' health screening programs using simple laboratory tests," he noted.